Enhancing the hole injection ability of indium tin oxide via ammonium salts in polymer light-emitting diodes

The enhancement of the hole injection ability of indium tin oxide (ITO) by spin-coating ammonium salts onto it is investigated. The work function of the ITO is increased via spin-coating an NH₄Br aqueous solution onto it, which is observed using ultra-violet photoelectron spectroscopy. The formation of H-bonds at the interface between the ITO and the NH₄Br is corroborated by utilising the surface sensitivity of synchrotron X-ray photoelectron spectroscopy at low photoelectron take-off angles. We illustrate the increase in the work function of the ITO with a possible scheme involving H-bonding between NH₄⁺ and the ITO. Polymer light-emitting diodes (PLEDs) with ITO/NH₄Br showed significantly enhanced performances compared to those with ITO only, with higher current density (924 mA cm^-2vs. 903 mA cm^-2) and brightness (44 224 cd m ^-2vs. 1987 cd m^-2) at a bias voltage of 7.0 V. To confirm the advantages of general ammonium salts for enhancing device performance, PLEDs with ITO coated by NH₄Cl, NH₄H₂PO₄, and (NH₄)₂SO₄ were fabricated and their performances were verified.